It appears to me that one of the basic questions regarding the possibility of
an urban dualmode system (and one which I have never seen addressed), is:

What is a reasonable minimum starter size for a true dualmode system - one
which can be considered a success in having an important congestion impact,
attracting sufficient numbers of users/vehicle buyers?

The value of any network - internet, cellphones, transportation, etc. -
increases with the square of the size of the network. Let's apply this same
thinking to an initial dualmode urban installation.

Case 1 - Assume a 12 mile long two-way system with four access/egress points
and/or stations, an average of four miles apart. Further assume that the
average overall cost is $10 million per mile. And assume that such a
"network", with its very limited capability, attracts 1000 "bare
bones"
vehicle buyers. The total system then costs $120 million; and vehicle
purchasers spend about $30 million (at an assumed cost of $30,000 per
vehicle). So such a very limited scale system implies total expenditures (or,
business opportunity) of about $150 million.

Case 2 - Assume a doubling of the network extent to 24 miles, with 8
"stations". This doubling in size implies that the number of vehicle
users/buyers quadruples to 4000. The system now costs $240 million and the
vehicles another $120 million, for a total expenditure of $360 million.

Case 3 - Double again to 48 miles and 16 stations. Quadrupling vehicle sales
then yields 16,000 buyers. Network cost becomes $480 million, and vehicle
costs are $$480 million, for a total expenditure of $960 million - almost $1
billion.

So what are the implications of considering the idea of different initial
scales of dualmode installation?

Where is a sufficient value point at which to begin consideration of
dualmode deployment?

What network configuration makes sense vs. network size?

How does the required capacity, and physical size, increase for each
access/egress/station as network size grows and the number of users (and
corresponding demand) grows?

Where do the funds come from? At what $ level must the feds get involved in
the funding question?

How much does the average cost per mile grow:
1) As demand and corresponding station size/throughput grows?
2) As installation and right-of-way acquisition costs grow?
3) As complex interchanges are added?
4) How much more difficult do dualmode operations become as complexity grows?
5) How do you prove the system will work safely/reliably, and attract
sufficient users/vehicle purchasers at each scale level?

I personally find it difficult to believe that a true dualmode system can
achieve sufficient value in a city unless it's at the scale of Case 4. At
this scale it's important for all in the buyer chain - government, citizens
and users - to be able to assess such important details as: "stations"
configurations; interchange configurations/sizes, operations details,
physical layout, capital costs, operating costs and user costs.

The big issue is: How does the general concept of dualmode ever get from
being "just another promising idea" to an actual deployment? Lots of time and
funding are required! Importantly, a process must be in place at the city,
state and/or national level for serious consideration of the overall concept
of dualmode. This process, and an accompanying organization, does not yet
exist, and is an urgent starting point if any dualmode idea is to eventually
become reality.